In fire safety systems, control panels are commonly employed to control devices such as smoke detectors via a two-line connection. From the two-line connection such devices will derive power, transmit and receive communication signals from the control panel. Intervening circuitry, comprised of a full-wave rectifier including four diodes, is generally connected to the two-line connection and is operatively located between the control panel and each device in the system to extract power and signals from the two-line connection to the sensor. The problem with such full-wave rectifier circuitry is that when the diodes in the rectifier are isolated, stray noise gets inside of diode bridge, producing a bias that causes signals between devices and the control panel to be lost.
Another problem with such rectifier circuits is that a soon as the rectifier circuit begins to develop a signal, the ground point of the circuit can become more negative that the diodes in the circuit, which may cause the diodes to stop conducting, disconnecting the circuit. This problem is magnified when any lines from monitoring or measuring equipment are connected to the rectifier circuit. These lines may pick-up noise and this noise may directly couple with internal capacitances of the circuit, driving the ground of the circuit even further negative.
A further problem with known systems where two-line connections are provided, from a control panel to one or more devices, is that due to human failure, the lines are commonly reversed, causing additional time and expense to correct the polarity reversal. Typically, such systems are polarity sensitive, so that they can be installed in only one manner. In order to reduce system commissioning time and expense, it is desirable to have an automatic polarity sensing power and signal interface between the control panel and the devices to eliminate the need to correct the polarity reversal.
It is therefore a primary object of the invention to provide a rectifier circuit with limited susceptibility to noise. It is a further object of the invention to provide an automatic polarity sensitivity power and signal interface.
These and other objects are achieved in accordance with the invention herein described. In fire safety systems, smoke detectors operate commonly with a two-line connection to a control panel. On these two lines the smoke detector derives power and transmits/receives communication signals. The present invention implements a full-wave rectifier comprising, in the preferred embodiment, a pair of diodes and a pair of transistors. The disclosed circuit automatically detects a polarity reversal at the two-line connection and still provides a proper output. The disclosed full-wave rectifier and accompanying circuitry below, provides a polarity insensitivity feature when two input lines are reversed, without requiring the use of logic circuitry. Further, the pair of transistors provides a low-impedance ground potential, substantially limiting noise within the rectifier circuit. The pair of transistors are used with the disclosed rectifier circuit to provide a low impedance path back to the internal ground from whichever of the two lines on the two-line connection is negative. The full-wave rectifier with two transistors and two diodes provides a low distortion output signal with a polarity insensitivity feature when the two input lines are reversed. As the transistors in the rectifier circuit provide resistance in both directions, noise is further limited since a path for noise is always provided between the internal ground and the negative line.
One object of the present invention is to provide a fire safety system having a control panel comprised of one or more controllers for controlling a plurality of devices which report system events back to the control panel. Preferably, a pair of lines are operably connected to the control panel and the plurality of devices for communicating signals between the control panel and the devices. At least one rectifier circuit may be operatively connected between the pair of lines and at least one device, such that the at least one rectifier circuit is comprised of at least a first transistor and a second transistor. Each transistor provides the rectifier circuit with a low-impedance ground path when turned on. The transistors further provide the circuit with a polarity insensitive output. Preferably, a sensing circuit is operatively connected between the rectifier circuit and at least one device. Each device may be provided with a device controller. Together, the sensing circuit and the device controller extract a signal from the rectifier circuit to be received by the device.